Is it forbidden for a man to marry his son's daughter or his daughter's daughter (granddaughters)?
Chapter on What is Prohibited to Marry and Combining Between Them and Other Matters
Al-Mughni
Book of Marriage
Primary text
It is forbidden for a man to marry his son's daughter or his daughter's daughter. This ruling is affirmed by the general consensus of the jurists, deriving its authority from the categorical prohibition regarding daughters stated in the Quran (Quran 4:23), which extends to the immediate female descendants.
Supporting text
Malik and the established opinion of Al-Shafi'i permit this marriage because the granddaughter is deemed unrelated (*ajnabiyyah*) to him legally, as the lineage does not legally trace through the daughter or son in a manner that invokes the prohibitive kinship ruling in this context.